Does this happen on a specific place or action?
This remains me a cookie tz_offset (and not offset) that has a value like that.
I don't know what this is cookie is for. In tiki-js.js, you can see
// Set client offset (in minutes) to a cookie to avoid server-side DST issues
// Added 7/25/03 by Jeremy Jongsma (firstname.lastname@example.org)
... just an idea